Inkjet printers operate a plurality of inkjets in each printhead to eject liquid ink onto an image receiving surface. In a tandem inkjet printing system, two different inkjet printing engines form printed patterns on a print medium, such as media pulled from a supply roll or other print medium. In some configurations, the tandem print engines form printed images on opposite sides of the print medium for duplex printing, while in other configurations an upstream tandem print engine forms a first printed image or portion of a printed image while a downstream tandem print engine forms another image or portion of an image over the images from the upstream print engine. In either configuration, the print engines move the print medium past inkjet printheads in separate print zones to form the printed images. The quality of printed images in the tandem printing configuration can be affected by comparatively small variations in the velocity of the print medium. Larger variations in the velocity of the print medium can result in registration errors where ink drops from different inkjet printheads in the print zone land in incorrect locations and fail to produce printed images having appropriate image quality. Consequently, the media transport devices in the tandem printer attempt to minimize changes to the velocity of the media web during the printing to maintain the quality of printed images.
In addition to controlling the velocity of the print medium, the tandem printing system includes a loop box that maintains a predetermined level of the position of the media web in the loop box. The loop box includes a vacuum in the base of the loop box or a payload weigh that increases or decreases the position of the bottom of the media web in the loop box as needed by increasing or decreasing the media web speed in either upstream engine or downstream engine. The limited range of motion for the bottom of the media web in the loop box can limit the effectiveness of the loop box in situations where a persistent difference in the velocity of the print medium persists through the upstream print engine and the downstream print engine. For example, if the velocity of the print medium through the upstream print engine is consistently higher than the downstream print engine, the print medium eventually goes slack between the two print engines. In another situation, if the velocity in the downstream print engine is consistently higher than the velocity in the upstream print engine, then the print medium experiences gradually increasing the bottom of the media web in the loop box and the media web eventually breaks.
Comparatively small differences in the velocity of the print medium between the upstream and downstream print engines can eventually overwhelm the loop box and require printer shutdown to adjust the media web. One source of velocity variation is due to the varying thicknesses of different types of print media that engage drive rollers and other rollers in the media transport paths of the upstream and downstream print engines. Varying thicknesses of different types of paper have a small but measurable effect on the velocity of the media web in different sections of the tandem printing system.
Additionally, even a single type of paper with a predetermined thickness can experience shrinkage, stretching, or other types of deformation that change the effective thickness of the paper in the printing system. Existing solutions that compensate for the thickness variations of different types of print media require a calibration process that is performed using the same type of paper before a printing operation can commence using the selected paper. The calibration operation consumes time and renders a portion of the print medium unsuitable for printing. Consequently, improvements to the operation of tandem printing systems that enable operation with a wide range of print media types without requiring offline calibration would be beneficial.